The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for MRI in which the magnetic field of the MRI scanner can be rapidly ramped up and down as needed.
MRI systems typically utilize one of two types of magnet assemblies to generate the strong, main magnetic field used for imaging. One type generates the main magnetic field using permanent magnets. This type of system is less popular because the magnetic field strengths that can be achieved with such systems is limited. Moreover, these systems tend to be extremely heavy and are very sensitive to temperature fluctuations. Permanent magnets also cannot be turned off, so there is no way to remove the magnetic field.
The second type of MRI system generates the main magnetic field using a superconducting electromagnet. Using superconducting magnets allows high current densities through the conductors of the electromagnet without power dissipation, which in turn enables the ability to achieve high magnetic field strengths. For the magnet to be superconducting, the magnet coils must be cooled to extremely low temperatures (e.g., about 4 K).
One method used to cool the superconducting magnet coils to this low temperature is done by immersing the conductor in a liquid helium bath. These superconducting systems tend to be very expensive because of the high cost of the liquid cryogens (e.g., liquid helium). Furthermore, it is not easy to rapidly turn on or off the magnetic fields generated by these systems. For example, to rapidly turn off the magnetic field typically requires heating up the conductive magnet coils so that they develop resistance that can dissipate their stored energy. This resistance produces heat that causes the liquid cryogen, which is providing the cooling, to convert to rapidly expanding gas. This boiling-off of the liquid cryogen removes the cooling capability of the system, and thus the magnetic field generated by the magnet coils. But, the magnetic field cannot be regenerated until the liquid cryogen is replaced and the magnet coils are cooled back down to superconducting temperatures, a process that normally involves multiple days and significant expense.
Alternatively, current can be removed or added to superconducting magnet systems very slowly without causing enough heating to boil off the liquid cryogen. In these situations, it takes many hours to completely add or remove the current, making rapid turning the magnetic field on or off in this manner not feasible.
For safety reasons, it would be beneficial for an MRI scanner to be capable of having the magnetic field rapidly turned off. For example, large metallic objects being attracted by the strong magnetic field is one of the primary risks associated with these devices. Traditional superconducting magnets have implemented a mechanism to rapidly turn off the magnetic field in an emergency situation by “quenching” the magnet in the manner described above, where all liquid cryogens are boiled off very rapidly. Quenching the magnet, however, requires a time consuming and expensive replacement of the liquid cryogens and before the magnetic field can be reestablished.
The ability to rapidly ramp up and down the magnetic field of an MRI system without the significant expense of losing and replacing expensive liquid cryogens would be very useful for interventional and mobile imaging applications. In these situations, it would be advantageous to ramp down the magnetic field of the MRI system so it could be safely stored (e.g., in a surgical suite) or transported, while at the same time allowing for the magnetic field to be rapidly ramped up (e.g., within a matter of minutes) for use as needed.